Method for coating metal substrates with a radically polymerizable coating agent and coated substrates

ABSTRACT

The invention relates to a method and coating agents used in the method which provide good adherence to and good corrosion protection of metal substrates; the coating coating agents are cross-linkable via radical polymerization. Adhesion promoting compounds are present in the conversion layer in a quantity of about 0.01 to 40 weight percent and contain, on average, one reactive unsaturated group and at least one H-active group per molecule.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. Ser. No. 11/151,713, filed Jun.13, 2005, which is a continuation of and claims priority fromInternational Application Number PCT/EP2003/013616, published in anon-English language, having an international filing date of Dec. 3,2003, and claims priority to DE 10258291.2 filing date Dec. 3, 2002.

FIELD OF THE INVENTION

The invention lies in the field of the anticorrosive treatment of metalsurfaces, which are coated with an anticorrosive conversion layer andthen with a radically polymerizable coating agent, the conversion layeracting as adhesive base for the subsequent coating agent layer.

BACKGROUND OF THE INVENTION

In the construction, domestic appliance and automotive industries, metalsubstrates are increasingly being used, and increased demands are beingmade of such substrates in terms of corrosion protection. In addition toknown hot galvanized or electrolytically galvanized steel sheets, coatedsurfaces with zinc, aluminum alloys are also used. Other metals,especially aluminum substrates or aluminum alloys, may also be employed.There is an extensive prior art relating to the deposition ofanticorrosive layers on metal surfaces. In the prior art, substantiallyaqueous solutions, which may optionally also contain organic polymers,are applied to the surface. The term conversion treatment indicates thatthe conversion solution reacts chemically with the metal surface duringthe treatment, with the formation of an anticorrosive layer, which maycontain metal atoms of the substrate and optionally polymers.

It is known that the use of chromium compounds or the addition ofchromate to the treatment solution results in a substantially betteranticorrosive layer and also in better adhesion. In principle, however,it is preferable, because of the physiologically harmful effect ofchromium, to avoid using this heavy metal in such processes.Furthermore, attempts are also being made to reduce other heavy metals,for example cobalt, copper, nickel, as far as possible. Thechromium-free conversion treatment of metal surfaces with fluorides ofboron, silicon, hafnium, titanium or zirconium in conjunction withorganic polymers to produce a conversion layer is known. Thepretreatment solutions are applied either by the rinse process or by theno-rinse process. In the rinse process, the conversion layer is rinsedafter it has formed; in the no-rinse process, the solution is appliedand dried without rinsing.

Accordingly, DE-C-24 33 704, for example, describes treatment baths thatmay comprise polyacrylic acid or its salts as well as ammoniumfluorozirconate. The conversion layers that form are said to exhibitimproved adhesion of the lacquer to subsequent layers. DE-A-197 51 153discloses a chromate-free coating for coil-coating steel sheets, inwhich titanium, manganese and/or zirconium salts of olefinicallyunsaturated polymerizable carboxylic acids and further olefinic monomersare applied together with initiators and are subsequently crosslinked byUV radiation. In both cases, further lacquer layers can subsequently beapplied.

WO 01/85853 describes a method of coating metal sheets with achromate-free UV-curing coating agent that comprises polyester acrylatesand polyurethane acrylates and that forms an anticorrosive layer on thesubstrate after crosslinking.

WO 01/32321 describes a method of coating metal substrates wherein asecond coating agent is applied to a first coating agent after curing,the second coating agent being curable by radiation. Before being coatedwith the coating agents, the metal substrate is treated with aconventional inorganic pretreatment solution in order to produce aconversion layer.

WO 02/24344 describes a method for the multi-layer lacquering of metalsurfaces in which at least one lacquer-like layer is applied to thesubstrate, wherein either the substrate is provided beforehand with ananticorrosive layer or no anticorrosive layer is used. The anticorrosivelayer and the lacquer-like layer are both crosslinked, UV-crosslinkingbinders together with photoinitiators being used for the lacquer-likelayer. The lacquer-like layer may optionally additionally also comprisecorrosion inhibitors and/or conductive particles.

A problem with the use of conversion protective layers and UV-curingcoating agents is the adhesion of the coating agent to the metalsubstrate. In the case of chromate-free pretreatments in particular,adhesion is often poor. In addition, even slight defects in the adhesionor cracks in the coating lead to increased corrosive attack on theunderlying metal substrate. Such cracks and defects of adhesion canoccur, for example, directly in the process of curing by actinicradiation as a result of shrinkage, or when the metal substrates aremechanically formed. Difficulties in terms of adhesion are encounteredespecially when the corresponding substrates are in the form of sheetsor strips and are to be protected against corrosion and coated in thatform, because such strips or sheets must subsequently be cut and broughtinto the appropriate later form. A further problem lies in the coatingof coil-coating materials themselves because, due to the process, onlyvery short reaction times are available in this application. It isfurthermore desirable to keep the number of successive method steps aslow as possible. The pretreatment solutions are generally acidicsolutions, so that the adhesion problems cannot simply be solved by theaddition of known adhesion agents, for example silane compounds.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a method ofapplying, in the coating of metal substrates, especially of metalsubstrates in sheet or strip form, a rapidly curing coating which hasthe good anticorrosive properties of known conversion layers and is freeof chromate, and which exhibits improved adhesion and a high degree offormability, especially in mechanical working of metal substrates.

One object is achieved by a method for coating metal substrates, inwhich an aqueous chromate-free conversion agent is applied to a cleanedmetal surface and a conversion layer is produced that contains from 0.1to 500 mg/m² of low molecular weight compounds which are crosslinkableby radical polymerization and additionally contain an H-functionalgroup. A coating agent that is curable by radical polymerization isapplied to the conversion layer, and the conversion layer and thecoating agent are then radically crosslinked.

The invention further provides a coating agent for producingchromate-free conversion layers on metal substrates, which coating agentis in aqueous acidic form and additionally comprises from 0.01 to 40 wt.% of low molecular weight compounds which can be crosslinked by radicalpolymerization and contain a further H-functional group.

The invention further provides a two-layer coating on metal substrates,which coating consists of a conversion layer according to the inventionand a further coating layer. The invention further provides a method forcoating substrates in sheet or strip form with a conversion layeraccording to the invention and a coating agent that can be crosslinkedby actinic radiation.

DETAILED DESCRIPTION OF THE INVENTION

Suitable substrates for the method according to the invention are metalsubstrates. They may be homogeneous substrates of one composition, butit is also possible for different types of substrate to be present nextto one another. The metal surfaces generally consist of aluminum, iron,copper, magnesium, nickel, titanium, zinc or of alloys of these metals.It is further possible to use conventional iron substrates that havebeen coated, for example, with zinc, aluminum, magnesium or alloysthereof.

The substrates may be flat or they may have a three-dimensional shape.However, the method according to the invention is suitable especiallyfor providing metal substrates in sheet or strip form with a surfacecoating, for example in a coil-coating process. The substrates can thenbe brought into other forms later.

As a first step in the method according to the invention, achromate-free conversion layer is produced on the prepared surface, i.e.a generally dust- and grease-free cleaned surface. The conversion layershould comprise from 0.1 to 500 mg/m² of coated surfaces of lowmolecular weight compounds that are crosslinkable by radicalpolymerization and additionally contain H-functional groups, preferablyup to 250 mg/m², especially from 0.5 to 100 mg/m². In particular, nopolymerization initiators should be present in the conversion layer.

A preferred possibility for producing the conversion layer is the use ofan aqueous chromate-free conversion solution comprising from 0.01 to 40wt. % of low molecular weight compounds that are crosslinkable byradical polymerization and contain H-functional groups. Aqueousconversion solution is to be understood as meaning the known aqueoussystems, which are to include molecular solutions, dispersions and/oremulsions. It is preferably applied at a temperature of approximatelyfrom 10 to 75° C. The temperature range should especially be from 15 to50° C. The aqueous conversion solution can be applied to the metalsurface by spraying, squeezing, immersion, using a roller, by means of aroll coater or a spray nozzle. Coating by means of a roll coater ispreferred especially for flat substrates. Methods of applying suchcoating agents are described in the literature and can be adaptedaccording to the intended use. The aqueous solution is generally allowedto act on the metal surface for a period of from 0.1 second to 5minutes, especially from 0.5 second to 30 seconds. It is preferable forthe operation to be carried out using the no-rinse process, i.e.subsequent rinsing of the conversion surface is not necessary.

The resulting conversion layer is initially not crosslinked further. Itis only very thin, and the layer thickness of the wet film should beless than 5 μm, especially less than 1 μm. The wet film is subsequentlydried, optionally also at elevated temperature. The conversion layerproduced according to the invention should contain amounts of radicallypolymerizable compounds. The material coated with the conversion layeraccording to the invention can be processed further directly oralternatively can be stored.

It is further possible, in order to produce the conversion layercontaining radically polymerizable compounds, to produce a conventionalchromate-free conversion layer on the substrate surface. This layer issubsequently dried or coated wet-on-wet with an aqueous solutioncomprising from 0.1 to 40 wt. %, preferably from 0.5 to 25 wt. %, of alow molecular weight compound containing at least one reactivepolymerizable double bond and at least one reactive H group, as well ascommercially available ionic or non-ionic emulsifiers, optionallyamounts of hydrophilic solvents and further auxiliary substances.Molecular solutions, emulsions or suspensions are to be included. Thecoating agents can be applied by the mentioned methods, but it is alsopossible to bring the aqueous solution into contact with the conversionlayer in the form of a simple rinsing bath.

A further procedure consists in reversing the sequence of the steps. Inthis case, the above-mentioned aqueous solution is applied to theoptionally cleaned substrate and is optionally flashed off, and then aconversion agent known in principle is applied. The conversion layer soobtained also contains a sufficient amount of low molecular weightpolymerizable compounds.

In a second step of the method according to the invention, coatingagents that can be cured by radical polymerization are applied. Suchcoating agents are widely known in the literature and have beendescribed for various purposes. They may be clear lacquer layers orpigmented systems. Liquid or pulverulent coating agents can be applied.It is possible to provide the corresponding layer as a primer withparticular properties; for example, conductivity, weldability, corrosionprotection, or decorative lacquer layers can be produced. Such systemsconsist, for example, of at least one radically polymerizable polymer,prepolymer or oligomer having unsaturated groups, reactive diluents ormonomeric components, as well as auxiliary substances conventional inlacquers. Such auxiliary substances may be flow agents, solvents,adhesion promoters, thixotropic and anti-settling agents, pigments orcolorings and, especially, polymerization initiators. In particular,prepolymers based on (meth)acrylate resins, optionally with furthercomonomers, have been found to be suitable. The initiators can beactivated by heat, or they are photoinitiators. Examples of suitablelacquer systems curing by polymerization are described in DE-A 199 56659, DE-A-199 25 631, U.S. Pat. No. 5,987,727, WO 2001091926 or EP0783534.

The coating agents to be applied in the method according to theinvention can be selected in view of the subsequent use. When thesubstrates are formed, flexible coating layers that do not crosslink toogreatly are preferably advantageous. The layers may be colorless, inorder to keep the substrate visible, but they may also be colored bymeans of colorings or pigments. In addition, particular additives can beused for specific applications, for example conductive and/oranticorrosive pigments. The coating agents comprise initiators, whichare commercially available products sufficiently well known to theperson skilled in the art. These coating agents are crosslinked byradical polymerization. If suitable initiators are used, polymerizationcan be carried out by means of heat. In the method according to theinvention, however, preference is given to coating agents that comprisephotoinitiators and can be crosslinked by actinic radiation.

The pretreated substrate surface is coated with a coating agent as asecond layer by known methods. It must be ensured that the conversionlayer formed in the first method step is not yet crosslinked. It shouldbe dry; optionally, it may also have just started to dry and can thus becoated with the coating agent by the wet-on-wet method.

Application of the coating agent according to the invention is generallycarried out at temperatures of from 10 to 90° C., preferably from 15 to75° C. The coating layer can be crosslinked by means of heat;preferably, the coating is crosslinked by actinic radiation, such as,for example, UV radiation, electron radiation or γ radiation. Electronradiation should have energy values of from 50 to 500 keV. Preference isgiven to crosslinking by UV radiation, especially with a wavelength offrom 150 to 800 nm, particularly preferably from 200 to 450 nm.Corresponding radiation sources are known to the person skilled in theart. The radiation intensity and the irradiation time depend on themethod conditions, for example the distance of the radiation source orthe relative movement between the source and the substrate. However, thetime is generally less than 60 seconds, preferably from 0.001 to 30seconds. The system parameters can be determined by the person skilledin the art by adaptation.

The layer thickness of the coating of conversion layer and coating agentthat is to be cured is from 0.1 to 100 μm, preferably up to 20 μm,particularly preferably from 0.5 to 10 μm.

In the preferred case of coating by coil-coating, these are applied, forexample, by spraying, squeezing, immersion, using a roller, by means ofa roll coater or a spray nozzle. Coating by means of a roll coater isparticularly preferred. The methods are known and can be adaptedaccording to the installation that is used. In this embodiment, theindividual method steps can be carried out immediately in succession.Crosslinking of the two layers is produced by actinic radiation,especially UV radiation. Once the coating has been crosslinked, thesubstrate can either be processed further directly or it is stored, forexample in the form of wound coil.

The conversion layer can also be produced using a coating agentaccording to the invention. Aqueous solutions for producing a conversionlayer are generally known and are described in the literature. Forexample, such solutions and application methods are described in EP-A694 593, EP-A 792 922, U.S. Pat. Nos. 5,344,504, 5,427,632, WO 95/14117or WO 95/04169. These are aqueous solutions, which generally have a pHbelow 6. They contain, for example, complex fluorides of Hf, B, Ti, Zr,Si, for example as hexafluorozirconic acid, hydrofluoric acid,hexafluorotitanic acid, hexafluorosilicates. The corresponding fluoridesof the mentioned elements can be present in the aqueous solution in theform of fluoric acids or alkali metal and/or ammonium salts. Phosphate,manganate, vanadate, tungstate or molybdate ions and similar ions mayalso be present in the conversion solution.

The aqueous solutions are generally stable at an acidic pH; the pH isespecially below 5. The aqueous solutions may comprise further auxiliarysubstances, for example reactive silanes, antifoams or wetting agents.Small amounts of organic polymers may further be present, for examplepoly(meth)acrylates, polyurethanes, epoxy resins, aminoplastic resins,phenolformaldehyde resins, polycarboxylic acids, polymeric alcoholsand/or esterification products thereof, polyvinylphenols, homo- orco-polymers containing amino groups, or copolymers of vinylpyrrolidone.Such polymers are known and are described, for example, in DE-A-100 10758.

The conventional known conversion solutions may be chosen as theprincipal constituent of the coating agent according to the invention.However, the solutions should be chromium- or chromate-free.

According to the invention there are present in the coating agent forproducing the conversion layer low molecular weight compounds thatcontain at least one group capable of crosslinking by radicalpolymerization. Low molecular weight compounds are to be understood asbeing liquid or solid, monomeric or oligomeric compounds, or mixturesthereof, which contain on average one or more double bonds reactive inthe polymerization, as well as a further H-active functional group.Polar and/or ionic components are contained in the molecule, andnon-polar hydrophobic components. Compounds having such double bondsare, for example, esters or amides of low molecular weight unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, crotonic acid,maleic acid, fumaric acid, etc., or polyester or polyether oligomershaving lateral or terminal (meth)acrylic groups, or other monomerscontaining activated double bonds, such as functionalized vinyl ethers.Preference is given to compounds having acrylate groups and/ormethacrylate groups. These compounds may contain a plurality ofreactive, polymerizable double bonds, but they preferably have at leastone reactive double bond. They generally have a molecular weight below1000. These compounds further contain at least one H-active functionalgroup, i.e. groups that are able to cleave protons. In the case ofesters or amides of unsaturated acids, the COOH groups of the acid maybe reacted with alcohols or amines which contain these H-active groupsas further functionality. Such H-active groups are, for example, OH, SH,NH₂, NRH, HOOC, HSO₃, HSO₄, (OH)₂PO, (HO)₂PO₂, (OH) (OR)—PO or (OH)(OR)—PO₂ groups. Particular preference is given to acidic H-functionalgroups, especially carboxylic acid, sulfonic acids, phosphonic acids andphosphoric acid groups. They are commercially available, for exampleCray Valley: SR 9050, SR 9051; UCB: Ebecryl 168, Ebecryl 170; Akros:Actilane 820; Rhodia: Sipomer PAM 100 and 200; Cognis: Photomer 4073,Photomer 2073; Rahn: Additiv 97-070, these products frequently beingmixtures.

The amount of low molecular weight reactive compounds should be from0.01 to 40 wt. %, preferably less than 25 wt. %, especially from 0.1 to15 wt. %, particularly preferably from 0.5 to 10 wt. %. One chosencompound may be present in the conversion solution, or it is possiblefor a plurality of different compounds to be present in the same ordifferent concentrations. In general, the compounds are mixtures ofmonomeric or oligomeric substances. The compounds are so chosen thatthey can be stably dispersed in the solution at the pH of the aqueousconversion solution. Their stability to phase separation or toprecipitation of their constituents should be more than 3 hours,especially more than 24 hours. This can be affected by the choice of thelow molecular weight compounds as well as by the pH value of thesolution. However, it may be advantageous to add a commerciallyavailable emulsifier or emulsifier mixtures, which should also be activein acidic solution.

The unsaturated double bonds should be capable of crosslinking byradical polymerization. This can occur by the addition of thermal orradiation-sensitive radical initiators. However, it is particularlypreferable for no radical initiators to be present in the coating agentsfor forming the conversion layer. The unsaturated double bonds shouldpreferably react by crosslinking with actinic radiation.

It is possible, when stable solutions are obtained, for the aqueoussolution for producing the conversion layer to be prepared in the formof a 1-component agent. This agent is usually prepared in concentratedform and then brought to a suitable viscosity and solids content withwater shortly before application. The unsaturated compound usedaccording to the invention is, then, contained in the concentrate. It isalso possible, however, if the monomers are not sufficiently stable inthe aqueous phase, to prepare a 2-component mixture. In this case, onecomponent should consist especially of the aqueous concentratedsolutions of a substantially known conversion agent, and the secondcomponent consists of the unsaturated reactive compounds, it optionallybeing possible for auxiliary substances, such as, for example, organicsolvents, preferably polar organic solvents, dispersion aids, antifoams,to be present proportionally or solely in the second component.Immediately before application, the two components are mixed andoptionally adjusted, with water, to a suitable application viscosity andsuitable solids content for application, and the coating agent accordingto the invention is then ready for use over a prolonged period. Mixingis carried out using known mixing devices.

The method according to the invention gives a coating that adheres verywell to the metal substrate and exhibits outstanding anticorrosiveproperties. With the selection of a suitable binder, a high degree offlexibility is obtained in further processing, and the coating does notbecome detached from the substrate. Good corrosion protection isobserved even at these formed locations. The use of systems thatcrosslink by actinic radiation, with a crosslinking step, ensures a goodbond to the substrate; short curing times can also be achieved, andcorrespondingly rapid processing can take place. It is found that energyconsumption is also lower than with heat curing systems.

If the coatings are processed by coil-coating processes, it isadvantageous to apply the coating agents using a roll coater. Individualsolutions may optionally also be applied to the substrate as a rinsingbath. It is thus possible to achieve multiple coating with a smalloverall length of the coating installation.

The conversion solution according to the invention exhibits goodapplication properties and can be adapted to the application conditions.Low foam formation is observed. Adhesion between the substrate and thecoating layer is improved and is ensured even after possible forming.

The substrates coated according to the invention are suitable for a widevariety of different applications, for example in the automotiveindustry, in the domestic goods industry and in the constructionindustry. The Examples that follow are intended to explain theinvention. It should be understood, however, that the invention is notlimited to the specific details of the Examples.

EXAMPLES Example 1

A pretreatment concentrate was prepared by mixing the indicatedcomponents (see Table 1 below, amounts are in wt. %). After intensivehomogenization, the solution was ready for use. It can, if required, bediluted for application with demineralized water.

TABLE 1 Example 1a Example 1b Demineralized water 64.6 46.8 ZnO 4.8H₃PO₄ (75%) 4.6 26.7 H₂ TiF₆ (50%) 15.0 H₂ TiF₆ (60%) 8.0Aminomethyl-substituted 20.4 polyvinylphenol Starch 0.3 Manganese(II)oxide 2.4 5.2 Molybdenum complex* 1.2 *according to U.S. Pat. No.5,683,816

Example 2

A UV-crosslinkable coating agent was prepared by mixing the componentsat room temperature (see Table 2, amounts in wt. %). The coating agentwas then ready for application.

TABLE 2 Example 2a Example 2b Urethane acrylate 34.9 35.5 Dipropyleneglycol diacrylate 19.5 8 Trimethylolpropane 34.3 34.9 formalacrylatePhosphoric acid ester 4.4 4.7 Bis-(2,4,6-trimethylbenzoyl)- 3.9 4.0phenylphosphine oxide 1-Hydroxycyclohexylphenyl 1.0 1.0 ketoneConductivity pigment 2.0 0

Example 3

12 g of a mixture of mono-, di- and tri-esters of the phosphoric acid of2-hydroxyethyl acrylate were added to 100 g of the solution from Example1a or 1b, and intensive stirring was carried out for 5 minutes.Distilled water was added to this mixture 1:1. The pretreatment solution(Example 3a, 3b) was then ready for application. The pretreatmentsolution exhibited lower foam formation than the solution of Example 1during processing.

The solution of Example 3a or 3b was applied to a dry HDG sheet-metalsubstrate which had been degreased with alkaline cleaners, and wasapplied with a roll coater. Drying was carried out for 5 minutes at 70°C. A coating agent according to Example 2 was then applied by means of aroll coater in a layer thickness of about 5 μm. The coatings were thenimmediately cured by irradiation with UV lamps (Fusion VPS/1600 type,H+D radiator, each 240 W/cm, 100% power) at a belt speed of 20 m/minute.T-bend and reverse impact tests were performed (see Table 3).

TABLE 3 Pretreatment Coating agent T-bend Reverse impact 3a 2a 3 1 3a 2b3-4 1-2 3b 2a 3 1-2 3b 2b 3-4 1-2

T-bend test: according to ECCA test method T7 [1996]: “Resistance toCracking on Bending”.

Reverse impact test: according to ECCA test method T5 [1985]:“Resistance to crack formation during rapid forming”.

Analogous tests were also carried out using cold-rolled metal sheets andwith aluminum as the substrate. The test results likewise showed animproved adhesion behavior of the pretreatment solutions according toExample 3.

Example 4

A degreased, dry HDG substrate was rinsed with a solution of 12 g of ahomogeneous mixture of mono-, di- and tri-esters of phosphoric acid with2-hydroxyethyl acrylate, 87 g of water and 1 g of a commerciallyavailable emulsifier. After brief evaporation of the water, a conversionlayer was produced on the surface from a solution according to Example1a. This conversion layer was dried for a short time and then coatedwith a coating agent according to Example 2a. The coating was cured asin Example 3. The coating adhered well to the substrate, and thecorrosion protection was good.

Example 5 Comparison Tests

The procedure of Example 3 was followed, but the pretreatment solutionaccording to Example 1 was used (see Table 4).

TABLE 4 Pretreatment Coating agent T-bend Reverse impact 1a 2a 5 5 1a 2b5 5

The comparison tests with the pretreatment solution without UV-curingcomponent showed markedly poorer adhesion behavior.

Example 6 Comparison Tests

0.12% of an aqueous dispersion ofbis-(2,4,6-trimethyl-benzoyl)-phenyl-phosphine oxide was added topretreatment solution 3a according to the invention, and the solutionwas applied as described in Example 3 to a cleaned HDG metal sheet, wasdried and then cured with UV radiation under the conditions likewiseindicated in Example 3. A coating agent according to Example 2a was thenapplied to the surface, and curing was again carried out with UVradiation (analogous conditions as previously). The coating exhibitedpoorer adhesion behavior (see Table 5).

TABLE 5 Pretreatment Coating agent T-bend Reverse impact 3a 2a 3-4 5

Although the invention has been described with reference to preferredembodiments, it is to be understood that variations and modificationsmay be resorted to as will be apparent to those skilled in the art. Suchvariations and modifications are to be considered within the purview andscope of the claims appended hereto.

What is claimed is:
 1. An aqueous coating agent for producing aconversion layer on metal substrates, said coating agent beingchromate-free and comprising: water; one or more complex fluorides of atleast one element selected from Hf, B, Ti, Zr and Si; at least one ionselected from phosphate, manganate, vanadate, tungstate or molybdateions from 0.01 to 40 wt. % of at least one low molecular weightoligomeric or monomeric compound which contains at least one H-activegroup that is an acidic group and at least one further functional groupcrosslinkable by radical polymerization, said oligomeric compound beingselected from polyester or polyether oligomers having lateral orterminal (meth)acrylic groups, said monomeric compound being selectedfrom esters or amides of low molecular weight carboxylic acids, or othermonomers which contain activated double bonds; wherein pH of the aqueouscoating agent is below 6; and wherein the low molecular weight compoundis present in an amount of 0.5 to 25 wt. % and comprises from 0.1 to 15wt. % of acidic derivatives of phosphoric acid or phosphoric acid with(meth)acrylates.
 2. The aqueous coating agent as claimed in claim 1,further comprising at least one auxiliary substance selected fromreactive silanes, antifoams or wetting agents.
 3. The aqueous coatingagent as claimed in claim 1, wherein the radically crosslinkablefunctional group is an unsaturated carboxylic ester or carboxamidegroup.
 4. The aqueous coating agent as claimed in claim 1, said coatingagent comprising from 0.1 to 25 wt.% of the at least one low molecularweight compound.
 5. The aqueous coating agent as claimed in claim 1,wherein the radically crosslinkable functional group is an acrylic ormethacrylic group.
 6. The aqueous coating agent as claimed in claim 1,wherein the low molecular weight compound is selected such that saidaqueous coating agent is stable against phase separation orprecipitation for more than 3 hours at the pH of the aqueous coatingagent.
 7. The aqueous coating agent as claimed in claim 1, wherein saidat least one element is present as fluoric acids, or alkali metal and/orammonium salts.
 8. The aqueous coating agent as claimed in claim 1,wherein said at least one element is present as hexafluorozirconic acid,hexafluorotitanic acid or hexafluorosilicates.
 9. An aqueous coatingagent for producing a conversion layer on metal substrates, said coatingagent being chromate-free and comprising: water; phosphoric acid;hexafluorotitanic acid; manganese (II); from 0.01 to 40 wt. % of atleast one low molecular weight oligomeric or monomeric compound whichcontains at least one H-active group and at least one further functionalgroup crosslinkable by radical polymerization, said compound beingselected from esters or amides of low molecular weight carboxylic acids,polyester or polyether oligomers having lateral or terminal(meth)acrylic groups, or other monomers which contain activated doublebonds; wherein pH of the aqueous coating agent is below 6, wherein saidat least one low molecular weight compound comprises a mixture of mono-,di- and tri-esters of the phosphoric acid of 2-hydroxyethyl acrylate.10. The aqueous coating agent as claimed in claim 9, further comprisingat least one of Zn, Mo and an aminomethyl-substituted polyvinylphenolpolymer.
 11. The aqueous coating agent as claimed in claim 9, furthercomprising a polyvinylphenol polymer.
 12. The aqueous coating agent asclaimed in claim 9, wherein no polymerization initiators are present inthe aqueous coating agent such that said at least one H-active group andat least one further functional group crosslinkable by radicalpolymerization remain uncross-linked after deposit on a metal substrateand remain cross-linkable by application of a second coating comprisinginitiator.